The invention relates to a liquid-crystal display device comprising two supporting plates having drive electrodes for applying a voltage across a layer of liquid-crystal material which is disposed between said supporting plates and which has a positive dielectric anisotropy and a wall orientation and twist angle of the liquid-crystal molecules, a birefringent effect at the lowest possible operating voltage being obtained by using said display device.
It is commonly known that in the case of perpendicularly incident light, a liquid-crystal display device, popularly termed TN-cell (Twisted Nematic), between crossed polarizers is not transparent to light when a sufficiently high voltage is applied across the cell. In this situation, the liquid-crystal molecules extend predominantly perpendicularly to the supporting plates, as a result of which the birefringent effect disappears almost completely and, hence, the crossed polarizers are not transparent to light. In practice, however, it has been found that in said situation such a cell always transmits some light, i.e. predominantly in the viewing directions the projection of which is located in the quadrant which is diagonally situated relative to the quadrant comprising the projection of the director in the centre of the cell on the front face, the axes of the quadrants being determined by the directors at the front and rear faces. The dark or non-transmissive state of the display device is poorly defined in said viewing directions, as a result of which they exhibit a much lower contrast than in the other viewing directions. When iso-contrast curves are polarly laid out as a function of the viewing angle (the angle with the normal) and as a function of the azimuth angle, it is found that said curves are not rotationally symmetrical about the normal. This phenomenon does not occur when the polarizers with their directions of polarization are arranged parallel to each other. Crossed polarizers are, nevertheless, preferred for most applications because they enable high contrasts to be obtained more readily than with parallel polarizers and because the dark state is colourless. The fact is, that when the polarizers are arranged in parallel, the transmission for the dark state of the display device is a function of the wavelength-dependent birefringence.
The picture quality of a display device of the TN-type could be considerably improved when the iso-contrast curves could be made rotationally symmetrical or, in other words, when the residual transmission in the non-transmissive state of the TN-display device could be equally reduced for all viewing angles.
This problem could be solved by means of a double cell construction, in which two identical TN cells are present between two parallel polarizers, which cells are arranged relative to each other in such a manner that the director at the front face of the second cell forms an angle of 180.degree. with the director at the front face of the first cell. A polarizer has to be provided between both cells, the direction of polarization of which extends perpendicularly to the direction of polarization of both other polarizers. When both cells are not driven, the combination is transmissive, the overall transmission being approximately 10% less than that of a single TN cell because the presence of a third polarizer introduces an additional light absorption. When both cells are driven simultaneously, the combination is, at all angles, not transmissive because the light which in this state leaks through one cell into the other is blocked. Such a construction is known from, for example, the Abstract of the published Japanese Patent Application 61-46930 (Patent Abstracts of Japan, Vol. 10, No. 206, p. 76, P478, Jul. 18, 1986). According to said Abstract, this construction leads to an improvement of a visibility characteristic which is not further defined in the Abstract.
However, the above-described and other display devices based on a double cell construction which are known from the literature generally have the disadvantage that the picture elements have to be individually defined and driven for each separate cell. A further important disadvantage is that the intermediate layer--in the known construction: two glass plates and a polarizer--is comparatively thick relative to the dimensions of the picture elements, so that the image on the display device has less sharp contours owing to parallax effects.